1. Field of Invention
The present invention relates to communications systems and, more particularly, to digital receivers used in digital communication systems.
2. Background
Communication systems use diversity transmission/combining techniques, i.e., transmitting and/or receiving the same signal over multiple propagation channels, to combat environmental effects such as multipath, fading, etc. If the same signal is transmitted by two transmitters or received by two receivers, it can be assumed that the noise in each channel is independent (i.e., there is no correlation between the noise in one channel and the noise in the other channel), whereas there is correlation between the two signals. This information can be used to combine the signals from each channel to achieve an improved overall signal-to-noise ratio.
Maximal ratio combining is a diversity combining technique used in receivers to provide improved sensitivity and diversity gain. Signals from each channel are weighted according to their signal-to-noise ratios, using their RMS (root mean squared) signal levels and noise variances, and added together. More specifically, the gain of each channel is made proportional to the RMS signal level and inversely proportional to the mean square noise level in that channel. Existing implementations that employ maximal ratio combining require either that the receiver(s) be in a single physical location or that the receiver(s) be in a known physical location(s) in order to support the signal processing to combine the signals. The known location information is used to synchronize and combine the incoming signals in order to get maximum diversity gain.